Proton (1H) Magnetic Resonance Imaging (MRI) works by imaging protons in the object and more specifically provides a representation of the specific chemical environment of the imaged protons. The information from the chemical environment of the protons can be acquired in many different ways. The protons are excited, and the information is acquired, by use of an electromagnetic pulse sequence.
The pulse sequence is designed to acquire specific information from specific chemical species containing proton nuclei. The most useful species in practice are fat and water. Therefore, several pulse sequences have been developed to acquire and display information only from water-containing tissues or only from fat-containing tissues. The resulting images are called chemical shift images. Other pulse sequences acquire data from both water- and fat containing tissues. The water-only images have widespread use in the clinical environment due to their ability to highlight the pathology in organs and structures without the interference from fat signal. On the other hand, water-plus-fat images are required for anatomic details.
So far, water-only and water-plus-fat images are normally acquired using two different imaging sequences. The increasing pressure in clinical practice to increase throughput, however, seldom allows the acquisition of multiple sequences, thus increasing the pressure to develop pulse sequences that will improve the ratio of information provided to imaging time used. This can be done either by decreasing the imaging time per sequence or increasing the information provided per sequence.
Several techniques have been developed to acquire water-only or fat-only images. In chemical saturation technique an additional long spectrally selective RF pre-saturation pulse is applied before the RF excitation pulse to suppress the signal of the undesired chemical species.
The chemical saturation technique is explained by Harms S E, Flaming D P, Hesley K L, et al. MR imaging of the breast with rotating delivery of excitation off resonance: clinical experience with pathologic correlation (Radiology 1993; 187:493-501); Haase A, Frahm J. Multiple chemical shift selective NMR imaging using stimulated echoes (J Magn Reson 1985; 64:94-102); Pauly J M, Nishimura G G, Macovski A, Multidimensional selective excitation (Proc Soc Magn Reson Med 1988; 7:654); Joseph, P M, A spin echo chemical shift MR imaging technique (J Comput Assist Tomogr 1985; 9:651-658); and Dumoulin C L, A method for chemical-shift-selective imaging (Magn Reson Med 1985; 2:583-585).
This technique, however, is sensitive to B0 and B1 inhomogeneities, and it also either increases the TR time or reduces the number of imaging slices. Another method for spectral selective imaging, which is gaining increased interest, is the three-point Dixon method.
The Dixon method is described by Glover G and Schneider E, Three-point Dixon technique for true fat/water decomposition with B0 inhomogeneity correction (Magn Reson Med 1991; 18:371-383); Totterman S, Weiss S L, Szumowski J, Katzberg R W, Hornak J P, Proskin H M, Eisen J, MR fat suppression technique in the evaluation of normal structures of the knee (Comput Assist Tomogr 1989 May-Jun; 13(3):473-9); Wang Y, Li D B, Haacke E M, Brown J J, A three-point Dixon method for water and fat separation using 2D and 3D gradient-echo (J Magn Reson Imaging 1998; 8(3): 703-710); Xiang Q S, An L, Water-fat imaging with direct phase encoding (J Magn Reson Imaging 1997; 7(6): 1002-1015); and Ma J, Wehrli F W, Song H K, Hwang S N, A single-scan imaging technique for measurement of the relative concentrations of fat and water protons and their transverse relaxation times (J Magn Reson 1997; 125(1): 92-101).
Although it provides water-only and fat-only images simultaneously and reduces the B0 inhomogeneity problem, the three-point Dixon method also has drawbacks. It increases imaging time, and limits the TE to specific values.
Water and fat images have also been obtained using spatial-spectral excitation, which is described by Schick F, Foster J, Machann J, Huppert P, and Claussen C, Highly selective water and fat imaging applying multislice sequences without sensitivity to B1 inhomogeneities (Magn Reson Med 1997; 38:269-274); Meyer C H, Pauly J M, Macovski A, Nishimura D G, Simultaneous Spatial and Spectral Selective Excitation (Magn Reson Med 1990; 15:287-304); Block W, Pauly J, Kerr A, Nishimura D, Consistent fat suppression with compensated spectral-spatial pulses (Magn Reson Med 1997; 38:196-206); Schick F, Simultaneous highly selective MR water and fat imaging using a simple new type of spectral-spatial excitation (Magn Reson Med 1998; 40:194-202); and Hardy P A, Recht M P, Piraino D W, Fat suppressed MRI of articular cartilage with a spatial-spectral excitation pulse (J Magn Reson Imaging 1998; 8:1279-1287).
In early studies, sinusoidally oscillating gradients and frequency-modulated RF pulses were used to obtain two-dimensional (2D) water and fat images, as described by Meyer, C H et al (cited above) and Spielman D, Meyer C, Macovski A, Enzmann D, 1H spectroscopic imaging using a spectral-spatial excitation pulse (Magn Reson Med 1991; 18:269-279).
With recent technology development, higher gradient amplitude and faster gradient switching rate has allowed the development of simpler and more efficient spatial-spectral excitations, as described by Shick et al, Block et al, and Hardy et (cited above) and by Thomasson D, Purdy D, Finn J P, Phase-modulated binomial RF pulses for fast spectrally-selective musculoskeletal imaging (Magn Reson Med 1996; 35:563-568).
Spatial-spectral excitations were implemented on 2D spin echo sequences to provide multi-slice imaging and on 3D gradient echo sequences (Shicik et al, Hardy et al, Thomasson et al). In the imaging of the knee, they provide better fat suppression than the conventional pre-saturation technique. However, this approach does not allow simultaneous acquisition of both water and fat images. In an effort to improve the ratio between information provided and imaging time used, simultaneous water and fat imaging techniques were developed to acquire water and fat data in alternate phase encoding line (Meyer et al, Schick et al). So far, the existing literature reports their implementation on 2D imaging only. 3D MRI allows the use of higher spatial resolution and also has a higher SNR. However, it requires longer acquisition time.
It will be readily apparent from the foregoing that a need exists in the art to be able to take and provide water-only, fat-only, and combined water-and-fat MR images rapidly and efficiently, without chemical shift artifacts or interference from other chemical species.
It is therefore a primary object of the present invention to acquire water-only and fat-only images during 3D image acquisition.
It is another object of the present invention to produce combined water-and-fat images from the acquired water-only and fat-only images.
It is another object of the present invention to produce combined water-and-fat images without chemical shift artifacts.
It is yet another object of the present invention to acquire water-only and fat-only images without interference from other chemical species.
To achieve the above and other objects, the present invention is directed to a system and method in which a pulse sequence contains alternating phase-encoding lines for the excitation of water and of fat within one data acquisition time. The pulse sequence provides water-only and fat-only images without interference from the other chemical species. It acquires and produces water-only and fat-only images during the same data acquisition time. It also produces 3-D water plus fat images with perfect local registration, eliminating the chemical shift artifact in both in-plane and through-plane directions, which none of the other sequences has been able to provide so far.
We designed and implemented a pulse sequence to simultaneously acquire water and fat 3D images in a single acquisition time, referred as three-dimensional interleaved water and fat image acquisition with chemical-shift correction (3-DIWFAC), for a 1.5 Tesla clinical MR scanner. In addition, since for clinical diagnosis, fat-suppressed and non-fat-suppressed images rather than water-only and fat-only images are used, we developed an algorithm to combine the water and fat images to produce water-plus-fat images. Furthermore, the algorithm was written in such a way that chemical-shift artifacts were removed.
The present invention makes use of spatial-spectral excitation to acquire water and fat 3D images simultaneously using the imaging time of a regular single acquisition. In addition, the water and fat images are combined to produce water-fat combined images free of chemical-shift artifact.